1. Field of the Invention
The present invention relates to a power supply apparatus provided with a power supply circuit for generating a DC output.
2. Description of the Related Art
A power supply apparatus such as an AC/DC converter, a DC/DC converter, a charger, etc. is popular in various fields. Generally, a power supply apparatus should be small in loss. Particularly, it is very important to reduce loss in a power supply apparatus used in a portable personal computer, a terminal unit in a mobile communications device, etc.
FIG. 1 shows the configuration of the charger or the DC power supply provided in the conventional power supply apparatus. The charger and the DC power supply have the same basic configurations, and each includes an electric power converter 510 and an analog circuit unit 520. In this example, the DC power supply is a DC/DC converter.
The electric power converter 510 includes a switching element (MOSFET) controlled according to the instruction from the PWM control circuit 524; a rectifying diode; an inductor for storing/discharging energy; a resistor for detecting an inductor current or an output current; and an output capacitor for smoothing an output. While the switching element is in the ON state, the inductor current ramps up with the electric current provided for the load, thereby storing residual charge in the output capacitor. While the switching element is in the OFF state, the inductor current ramps down and the electric charge stored in the output capacitor is discharged as necessary with the electric current provided for the load.
The analog circuit unit 520 includes an amplifier 521 for amplifying an inductor current; an amplifier 522 for amplifying the difference between the output from the amplifier 521 and a reference voltage Vref1; an amplifier 523 for amplifying the difference between the output voltage and a reference voltage Vref2; a PWM control circuit 524 for generating a PWM signal for controlling the switching element based on the output from these amplifiers, etc.; and an oscillator 525 for providing a clock at a predetermined frequency for the PWM control circuit 524.
When the output voltage drops below the reference voltage Vref2, the PWM control circuit 524 makes the duty (duty cycle) of the PWM signal to be provided for the switching element higher so that the inductor current can be increased and the output voltage will increase. On the other hand, when the output voltage becomes higher than the reference voltage Vref2, the PWM control circuit 524 makes the duty of the PWM signal lower so that the inductor current can be reduced and the output voltage will drop. Thus, the output voltage can be maintained at a constant level. When the PWM control circuit 524 detects an overcurrent based on the output from the amplifier 522, it reduces the duty of the PWM signal or forcibly turns off the switching element.
Thus, an analog circuit has been used to control the output from a charger provided in the conventional power supply apparatus, or each DC power supply.
As described above, an analog circuit has been used to control the output from the conventional power supply apparatus. Therefore, the characteristics or specification of a power supply circuit cannot be easily changed. If they can be changed, a number of circuits have to be added for amendments. Considering a smaller or a lower cost power supply apparatus, the conventional technology has been impractical and unrealistic. Described below are some of the problems with the conventional power supply apparatus.
(1) Precision in Output Voltage
To control and maintain the output voltage of a power supply circuit, the output voltage is normally used as a feedback signal. However, a transfer function for a feedback system changes with external factors (input voltage, output current, temperature, etc.). Therefore, if such factors are not taken into account, oscillation may occur in the feedback system.
Therefore, in the conventional power supply apparatus, an amplifier for the feedback system is designed for the worst possible case in order to avoid the above described oscillation. However, this design reduces a gain in the normal operation, thereby lowering the precision in control of an output voltage. This problem is due to the characteristic of an amplifier, which cannot be flexibly changed depending on various factor such as input voltage, output current, and temperature, in the conventional analog circuit.
(2) Error in Digitalizing Data
In newly developed technology, the functions of a conventional analog circuit can be replaced with digital control. For example, a power supply apparatus has been designed such that an output voltage as a feedback signal is converted into digital data from which numeral data for control of a switching element is generated, and then, the switching element is controlled according to the numeral data. However, under the control, an unavoidable digital error (quantization error) occurs. The error may cause a ripple in an output voltage.
To reduce the above described error, the quantization step should be reduced. However, reducing the quantization step raises the cost and increases the electric power consumption in the power supply apparatus.
(3) Power Consumption
When an electric current required by a load is small, for example, in the suspense mode of a personal computer, etc., it is particularly important to reduce the power consumption of a power supply apparatus itself. This technology has long been studied, but has not been satisfactorily developed, and therefore should be improved.